1. Technical Field
The invention relates to a driving voltage generator and a digital-to-analog converter thereof.
2. Related Art
In a display device, in order to present images with correct gray levels on a display panel, a source driver of the display device is required to provide a correct gamma voltage to the display panel according to gray level data to be displayed on the display panel. Therefore, a digital to analog converter (DAC) is used on the source driver of the display device to produce an analog driving output voltage according to digital gray level data, and the display panel is driven to display images according to the driving output voltage.
Referring to FIG. 1, FIG. 1 is a circuit diagram of a DAC 100 of the conventional technique. The DAC 100 includes switches formed by a plurality of transistors MP1-MP11. The switches formed by the transistors MP1-MP7 are grouped into three stages, and the transistors MP1-MP4 arranged in a first stage respectively receive four gamma input voltages VP[n]-VP[n+3], and the transistors MP1, MP3 and the transistors MP2, MP4 are respectively controlled by selecting signals SEL[n] and SELB[n], wherein the selecting signal SELB[n] is an inverted signal of the selecting signal SEL[n]. Moreover, the transistors MP5 and MP6 arranged in a second stage respectively receive a voltage on a common coupling node of the transistors MP1 and MP2 and a voltage on a common coupling node of the transistors MP3 and MP4. The transistors MP5 and MP6 are respectively turned on or turned off under control of selecting signals SEL[n+1] and SELB[n+1]. The transistor MP7 is coupled to a common coupling node of the transistors MP5 and MP6, and is turned on or turned off under control of a selecting signal SELB[n+2], and the transistor MP7 is used to determine whether or not to provide the voltage on the common coupling node of the transistors MP5 and MP6 to serve as a driving output voltage VOUTP.
According to FIG. 1, it is known that when any of the gamma input voltages VP[n]-VP[n+3] is transmitted to serve as the driving output voltage VOUTP, it has to passes through three transistor switches. Namely, in the conventional DAC 100, a transmission path from which the gamma input voltage is transmitted to serve as the driving output voltage VOUTP has a certain transmission resistance. Such transmission resistance increases as the number of the gamma input voltages increases, which may even influence the quality of the driving output voltage VOUTP. Moreover, in the DAC 100 of the conventional technique, the number of the required transistor switches increases as the number of converted bits increases, which greatly increases a required circuit area.